Methods in which images are reproduced by means of photographic techniques using photosensitive compositions comprising a mixture of polymerizable polymers which have ethylenic unsaturated bonds and polymerization initiators and, as required, binding agents which have the appropriate film forming ability and thermal polymerization inhibitors, are well known at the present time. In this regard, it has been disclosed in U.S. Pat. Nos. 2,927,022, 2,902,356 and 3,870,524, that such photosensitive compositions undergo photopolymerization and are hardened and rendered insoluble when irradiated with light. Thus, by forming this type of photosensitive composition into an appropriate film and irradiating the said film with light which has passed through the prescribed negative image, it is possible to form a prescribed hardened image of the photopolymerizable composition by removing the unexposed parts of the film with an appropriate solvent (a process which is referred to below as "development"). Photosensitive compositions of this type are unquestionably very useful, for example, in making printing plates.
In the past, photosensitivity was found to be inadequate when just polymerizable compounds which had ethylenic unsaturated bonds were used. Therefore, the addition of photopolymerization initiators was suggested as a means of raising the photosensitivity. Benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethylanthraquinone have been used for this purpose. However, when these photopolymerization initiators have been used, long imagewise exposures are required to form the image, because the hardening sensitivity and reactivity of the photopolymerizable composition is reduced. Consequently, it is difficult to reproduce fine images with good picture quality if there is any vibration during the operation. Also, consideration must be given to dispersing the large amounts of heat which are produced since the amount of energy radiated by the light source used for making the exposure must be increased. There are also problems concerning deformation and deterioration of the film which has been formed due to the heat.
Photopolymerization initiators typically have a much lower photopolymerization effect with light sources in the visible region above 400 nm than they do with light sources in the ultraviolet region below 400 nm. Therefore, photopolymerizable compositions which contain conventional photopolymerization initiators are limited in their range of application.
There have been some suggestions made in the past in connection with the development of photopolymerization systems which respond to visible light rays. For example, it has been reported in U.S. Pat. No. 2,850,445 that certain types of photoreducing dyes such as rose bengal, eosin, and erythrocine have effective responsiveness to visible light. Other attempts at improving the technique include the use of conjugate initiator systems consisting of dyes and amines (JP-B-44-20189) (the term "JP-B" as used herein signifies "examined Japanese Patent publication"), system containing a hexaarylbiimidazole with a radical generator and a dye (JP-B-45-37377), systems consisting of a hexaarylbiimidazole and a p-dialkylaminobenzylideneketone (JP-A-47-2528 and JP-A-54-155292) corresponding to U.S. Pat. No. 4,162,162 (the term "JP-A" as used herein signifies "unexamined published Japanese patent application"), systems consisting of 3-keto substituted coumarin compounds and active halogen compounds (JP-A-58-l5503 corresponding to U.S. Pat. No. 4,505,793), and systems consisting of substituted triazines and merocyanine dyes (JP-A-54-l5l024). These techniques have been effective with respect to visible light beams, but have yet to achieve a satisfactory photosensitive rate. Therefore, there is a need for further improvement of these techniques.
In recent years, increased sensitivity to ultraviolet light and methods of forming pictures using lasers have been investigated and UV projection exposure methods, for the production of printing plates, direct laser plate making, laser facsimilie and holography, have already reached a practical stage. Photosensitive materials of high sensitivity have been developed to correspond with these techniques. However, adequete sensitivity of these materials has yet to be achieved.